Can antibiotics cause colon polyps? In a landmark study from the Harvard School of Public Health and the Nurses Health Study, Cao and colleagues have shown that long-term antibiotic use is associated with a remarkable increased incidence of colorectal adenomas, otherwise known as adenomatous polyps, or simply colon polyps, the precursor lesions for colon cancer. The article was published in the journal Gut and can be viewed here. By way of background, the HSPH has published a large number of studies based on longitudinal observations of thousands of doctors (male) and nurses (female). Diet, use of vitamins and other parameters have been examined for at least twenty years. This study looked at more than 16,000 nurses over the age of 60. It looked back at their antibiotic usage starting at age 20. The relative risk of colon adenomas was markedly increased, and quantitatively increased based on the extent of antibiotic usage. Women who used antibiotics for more than two months between ages 20-39 had a relative risk of 1.36; between the ages of 40-59 the RR was 1.69. More recent use did not convey the same risk.
What is the basis for this increased risk? The microenvironment of the colon is determined to a great extent by its flora. There are trillions of bacteria in the average colon, and antibiotic use can change the types of bacteria found. Some bacteria incite more of an inflammatory response than others, perhaps the mediator for polyp formation. Sometimes the antibiotics are given for intestinal ailments, so there may be an underlying basis for the association, not just the antibiotic use per se.
What can we learn from this? Simply it is a reminder that in medicine there is no free lunch. Antibiotics are way overprescribed in the United States, and these results show one more reason that this is a bad idea.